Rod and S-cone signals have been shown to interact in an additive manner at threshold. S-cone ganglion cells receive color-opponent input without center-surround antagonistic structure, which is unlike ganglion cells that receive color-opponent input from L- and M-cones. Interaction between rods and S-cones may therefore depend on whether rod and S-cone modulation covers the same spatial location (local) or different spatial locations (lateral) (see review by Buck 2004).

Here, rod thresholds were measured at different phases for a rod pulse superimposed on an S-cone sinusoidally-modulated background. Rod and S-cone isolating stimuli (silent substitution) were generated with a 2-channel 4-primary Maxwellian view system. Detection thresholds for a 100-ms 1-deg rod-isolating pulse were measured against one of four possible adapting backgrounds: 1) a 12-deg steady full-field, 2) a 1-deg cone-modulating field (local), 3) a 12-deg cone-modulating annulus with a 1-deg inner diameter (lateral) or 4) a 12-deg cone-modulating full-field (local and lateral together) (Lee, Decay, Smith & Pokorny, 1999). The cone modulation was a temporal sinusoid along the S axis and did not modulate rods. The stimulus was presented 6-deg extra-foveal (nasal) at 200 Td and 20 Td.

Results show that, at 200 Td, rod thresholds varied with phase in a systematic way for all conditions tested. At 20 Td, however, the variation with phase was only observed for the full-field condition. This suggests that the spatial configuration of the adapting background affects the type of interaction observed, but differently to what is seen for rod and L-/M-cone interaction.